1. Field of the Invention
This invention relates in general to electromechanical alignment and isolation, and more particularly to a method and apparatus for supporting and aligning a wafer in a microlithographic system having extreme precision.
2. Description of the Prior Art
Various support and positioning structures are well known for use in microlithographic instruments. Typically in the prior art, XY guides including a separate X guide assembly and a Y guide assembly are utilized with one guide assembly mounted on and movable with the other guide assembly. Often a separate wafer stage is mounted on top of the guide assemblies. These structures require high precision in manufacturing and many components. These structures are typically used in a wafer stepper apparatus where the alignment of an exposure field to the reticle being imaged affects the success of the circuit i.e., the yield. In a scanning exposure system, the reticle and wafer move simultaneously and scan across one another during the exposure sequence.
A related system is disclosed in copending and commonly owned U.S. Pat. application Ser. No. 08/221,375 filed Apr. 1, 1994, titled xe2x80x9cGuideless Stage Isolated Reaction Stagexe2x80x9d invented by Martin Lee, now U.S. Pat. No. 5,528,118 issued Jun. 18, 1996, and copending and commonly owned U.S. Pat. application Ser. No. 08/266,999, filed Jun. 27, 1994, titled xe2x80x9cElectromagnetic Alignment and Scanning Apparatusxe2x80x9d, invented by Akimitsu Ebihara. See also U.S. Pat. No. 5,040,431 issued Aug. 20, 1991 to Sakino et al. and U.S. Pat. No. 4,667,139 issued May 19, 1987 to Hirai et al. All of the above patent disclosures are incorporated herein by reference. Many other examples of such stage structures, often called xe2x80x9cXY stagesxe2x80x9d, are known in the art.
Prior art stages typically suffer from a significant drawback in that the sensitivity of measurement accuracy of the stage position is adversely affected by temperature. The electromagnetic motors which drive the elements of the stage relative to one another are a significant heat source adversely affecting the performance of the laser interferometry typically used to determine the actual stage position.
Another disadvantage of prior art systems is that the numerous cables including electrical cables, fiber optic cables, coolant tubes, vacuum tubes and air hoses connecting to the stage from external devices impose a significant amount of drag and mechanical forces, both steady and impulsive, on the actual stage, thus degrading performance. Thus, cable drag occurs as the stage moves about pulling the cables with it, causing thereby mechanical friction and disturbances.
Additionally, prior art stages suffer from reduced performance due to the relatively high mass of the stage which typically carries the heavy electromagnetic drive motor magnets for positioning the stage in at least one axis direction. Higher mass may reduce the stage mechanical resonance frequency and thereby lower the stage performance. If the stage is made stiffer to compensate, this may add even more mass. Higher mass requires more motor power, thus undesirably more potential for heating.
Therefore, there is a significant problem in the prior art of impeded stage performance in terms of accuracy and speed caused by the relatively high weight of the stage support, the cable drag, and the heat generated by the stage movement impeding sensing accuracy in terms of position.
In accordance with the invention, an XY stage apparatus is capable of high accuracy positioning and motion control in three dimensions. The apparatus uses linear commutated motors to drive the main stage in a two-dimensional plane and controls out-of-plane linear motion (along the Z axis) as well as rolling and pitching rotations through the use of air bearings.
The main stage in one embodiment straddles a beam (guide bar) that is mechanically driven on a base by linear motors in a first linear direction. A follower stage (follower frame), not mechanically connected to the main stage, also moves independently in the first linear (X) direction between fixed guides mounted on the base, and its motion is synchronized to the main stage motion in the X direction. As the main stage and follower stage move independently but simultaneously in the X direction, linear electromagnetic motors, the magnetic tracks of which are mounted on the follower stage and the coil portions of which are mounted on the main stage, move the main stage in a second linear Y direction normal to the X direction.
Thereby the main stage is isolated from mechanical disturbances in the XY plane and control of accuracy of movement of the main stage is improved by removing the weight of the magnetic tracks from the main stage itself.
Additionally, the sensitivity of measurement accuracy of stage location to temperature effects is improved by minimizing the number and size of heat sources in close proximity to the main stage and the measuring laser beam paths which are part of the interferometry measurement system.
Air circulation is provided through the slots in the magnetic tracks in which the motor-coils ride. These slots are partially sealed at either end of the main stage so as to cause air flow through the tracks which is ducted away from the main stage, and thus away from the interferometry laser beam paths.
Additionally, a cable follower stage is mounted on the follower stage. The cable bundle is connected from the main stage to the cable follower stage. The cable follower stage moves in one dimension along the follower stage in synchronization with the movement in that direction of the main stage, and thus supports the bulk of the weight of the cable bundle (including electrical and optical cables, air and vacuum tubes) connected external to the apparatus.
Thus, high accuracy and movement is achieved by obtaining optimum control, minimizing thermal effects, and substantially eliminating cable drag.
The apparatus in accordance with the present invention is suited for use as a wafer stepper and scanner in a scanning exposure system by providing smooth and precise stepping and scanning in two-dimensions. Additionally, the present apparatus is adapted to a scanner system wherein the Y direction is the scan direction and the X direction is the cross scan direction.
Advantageously, precision, accuracy, acceleration, velocity and settling time are improved over the prior art. The main stage and its supporting beam (movable guide bar) are advantageously reduced in weight because the relatively heavy magnetic tracks for driving the stage are mounted on the independent follower stage instead of on the beam. The forces applied to accelerate and decelerate the main stage are effectively applied at or near the center of gravity of the stage. This advantageously reduces torque moment on the stage and thereby reduces a tendency for rolling and pitching of the stage. Thus the control in the X and Y directions is optimized.
Additionally, use of the beam for driving the main stage reduces the number of heat sources located on the stage, thereby reducing thermal effects on the interferometry system which determines stage location.
Further use of the follower stage to drive the main stage in the Y direction means that the linear motor coil, often located in the prior art at the center of the main stage, is replaced by two motor coils at two edges of the main stage, each motor coil requiring only one half of the power compared to the use of a single prior art drive motor to achieve similar movement. This not only physically locates the heat source away from the wafer (which is located at the center of the main stage), but also reduces the concentration of heat generated, thereby facilitating a limitation of thermal effects on the stage and on interferometry positioning.
The cable follower stage serves as an intermediate resting place for the cables between the main stage and the external cable connections, the cable follower stage thereby being a mechanical buffer between the main stage and mechanical disturbances due to cable drag. Thus precise movement of the stage is enhanced.
Advantageously, an apparatus in accordance with the invention uses commercially available electromagnetic drive motor components rather than the special commutatorless electromechanical drive elements used in some prior art stages. Thus, the apparatus is relatively easily manufactured and cost is reduced. Further, the heat generated is substantially reduced over that of the prior art commutatorless motors, reducing thermal effects.
The stage need not surround or even straddle the beam, so long as the beam can mechanically drive the stage. In one embodiment, a linear motor coil is located at each end of the beam and drives the beam along magnetic tracks located on the guides elongated in the X direction and fixedly mounted on the base. Thus, the motion of the beam in the X direction (cross scan direction) is transmitted to the main stage because the main stage is coupled to the beam. The beam also serves as a guide for the main stage in the Y direction (scan direction) as the main stage slides freely along the beam via air bearings disposed on surfaces of the main stage facing the beam. The driving motion of the main stage in the Y direction is provided by two linear motor coil assemblies, each mounted on one of two parallel edges of the main stage. The two coil assemblies move inside the magnetic tracks mounted on the follower stage which is located surrounding but mechanically independent of the stage.
The follower stage is a rigid rectangular structure including two parallel members (called herein bridges) that connect two other parallel members at right angles. Two linear motors move the follower stage along the guides in synchronization with the main stage and the beam motion in the X direction.
To ensure smooth sliding motion of the moving members (main stage, beam, follower stage) with respect to the principal surface of the base plate, air (or other fluid) bearings are located underneath the main stage, the beam, and the follower stage. Additional air bearings are also disposed between the follower stage and the fixed guides for smooth follower stage motion in the X direction. Air bearings disposed between the main stage and the beam ensure smooth relative motion of the stage in the Y direction. In one embodiment only one fixed guide is provided, and so the beam and follower frame are guided only at one end.
The follower stage is guided in the X direction and restrained in the Y direction by two opposed pairs of air bearings which clamp to and glide along one of the fixed guides. The beam is guided in the X direction by a combination vacuum and air bearing which moves along the other fixed guide.
Alternatively in another embodiment no air bearings are disposed between the beam and the fixed guides. Instead, air bearings are disposed between the beam and the bridge portions of the follower stage for restraining the beam in the Y direction. Because the beam motion and the follower stage motion are synchronized in their respective X direction movements by an electronic control system, relative motion (only a few millimeters) is limited between the beam and the follower stage and only arises from the lack of perfect synchronicity in the control system between these two elements. Thus bearing mechanical noise at this location is minimized.
xe2x80x9cAir plugxe2x80x9d structures and corresponding air ducts improve heat removal from the linear motors that drive the stage in the Y direction along the beam. The slots which accommodate the magnetic tracks on the follower stage are closed at each corner of the main stage by air plug structures so as to contain the warm air inside. These air plugs are formed in the shape of the interior portion of the slot in cross section and have a similar clearance to that of the motor coil relative to the magnetic track. Thus there is a small gap between the air plug structures and the magnetic track interior. Thus the air plug structures do not completely seal the air movement in the slots, but form sufficient restriction between the air inside the slots and outside to allow a slight negative pressure to develop. This ensures, in conjunction with a ducted vacuum outlet located in a center portion of the magnetic track, the controlled flow of air from the outside moving into the stage motor coil slots. This air in turn is removed by the ducts. Duct tubing is attached to the outlet in the magnetic track assembly to circulate air therethrough to cool the coil assemblies at its center portion.